The present invention relates to apparatus for the cryogenic distillation of air and, in particular, relates to a modular apparatus, methods of construction and transportation of said apparatus and use of said apparatus to distil air.
The present invention is of particular use in the construction of air distillation plants by the coast having dockside facilities including heavy duty lifting equipment. In addition, the present invention is primarily concerned with the production of oxygen from air distillation plants but, as would be readily appreciated by the skilled person, it is readily applicable to the production of other products from the cryogenic distillation of air.
A cryogenic air separation process typically comprises the removal impurities such as carbon dioxide and water from feed air. The purified air is then compressed and cooled to a cryogenic temperature and is then fed to a cryogenic distillation system in which it may be separated into oxygen products, nitrogen products and rare gas products such as argon, xenon and krypton.
The cryogenic distillation system may comprise a single distillation column but typically comprises a high pressure column thermally integrated with a low pressure column and may further comprise an auxiliary column, such as an argon side-arm column, if rare gas products are required.
In a dual-column system, the cooled compressed feed air is fed to the high pressure column where it is separated into an oxygen-enriched bottoms liquid and a nitrogen-enriched overhead vapour. At least a portion of the oxygen-enriched bottoms liquid is fed to the low pressure column after appropriate pressure reduction where it is separated into liquid oxygen and low pressure nitrogen overhead vapour. At least a portion of the nitrogen-enriched overhead vapour from the high pressure column is condensed by indirect heat exchange against liquid oxygen in a reboiler-condenser located in the sump of the low pressure column. At least a portion of the resultant liquefied nitrogen-enriched overhead vapour is fed back to the high pressure column as reflux for the distillation. A portion of the liquefied nitrogen vapour may be removed as product and liquid oxygen may be removed as product from the low pressure column. There are numerous variations of this process known to the person skilled in the art.
Construction of an air separation plant is complex and is usually carried out by specialist construction engineers. As a consequence, it is usually time-consuming and expensive. On-site construction has been simplified to a certain extent by the use of a modular approach in which modules containing components for the air separation plant are transported to the plant site and interconnected on-site. Examples of such a modular approach are disclosed in U.S. Pat. No. 5,461,871 (Bracque et al) and EP-A-1314942 (Stringer et al).
In the system disclosed in U.S. Pat. No. 5,461,871, the principle air compression components are housed in one module, the “warm” elements (other than those required for compression and purification) are housed in a second module and the “cold” elements such as the principle heat exchanger are housed in a third module. The second and third modules are of parallelepipedal shape whose external dimensions permit road transport. Alternatively, the second and third modules may be connected into a single module transportable by road. Once transported to the plant site, the modules are assembled with the other components such as the distillation column system. It is disclosed that such a modular system is suitable for plants producing tens of tons per day of oxygen.
EP-A-1314942 also discloses a modular system for the construction of air separation plants. In this system, first and second modules are selected from libraries of modules according to the design of the plant being constructed. The first module preferably comprises a high pressure distillation column and the second module preferably comprises a main heat exchanger. A low pressure distillation column may be contained within a third module selected from a third module library. The modules of each library each have a set of interface points that are arranged having substantially the same relative spatial co-ordinates thereby allowing each member of one library to be connected to each member of another library. The selected modules are transported to site where they are assembled into the plant. It is disclosed that this system may be used in the construction of plants producing 200-2000 metric tons/day of gas.
In existing modular systems, the modules still have to be assembled on-site which is both time-consuming and requires skilled engineers. There is a need, therefore, for a new air separation plant, the construction of which is simplified, less expensive and less time consuming than the construction of existing plants.
In typical air separation plants, the components are generally spread out over the ground within a defined area. Where space is restricted, it would be desirable to reduce the ground area taken up by the footprint of the air separation plant.
The components of an air separation plant are usually manufactured in one location and transported to the site of the plant for assembly. More often than not, the plant site is in a different country to the country of manufacture and, thus, the components are usually transported to the plant site by sea. As specialist construction engineers are required to assemble the plant, it is often necessary to send engineers abroad for considerable lengths of time. This is expensive and, in certain parts of the world, may even be dangerous to the health and safety of the engineers. There is a further need, therefore, for a new air separation plant which may be constructed at less cost and with less risk to health and safety of the engineers.
The site of the air separation plant could potentially be anywhere in the world. As the plant is usually transported to site in its component parts, there is a risk of contamination of the interior components of the plant which could have a significant deleterious effect on the operational efficiency of the plant. This is especially relevant to plants having one or more large distillation columns, e.g. columns having a diameter of over 3.5 m and usually about 5 m or 6 m. Such columns at present may be transported in sections and, thus, the inner surfaces of the column sections may be exposed. Contaminants include dirt and grease and may be airborne. Some contaminants may even be corrosive, for example salt, or erosive and/or abrasive, for example sand. There is a need, therefore, for a new air separation plant which may be constructed with less risk of contamination of the interior components and hence less risk of a reduction in the operational efficiency of the plant and associated operating costs.